1. Field of the Invention
The present invention relates to a time-division multiplexing radio system, and especially to the time-division multiplexing radio system that secures the linearity of a signal output to be transmitted.
2. Description of the Related Art
In general, in digital modulation radio systems that use a time-division multiplex technique, large amounts of nonlinear distortion of a signal amplified by a power amplifier incorporated in a transmitter will increase power to be leaked into adjacent channels. The larger the leaked power, the more deteriorated the modulation precision of a transmission signal which has large influence on transmission characteristics. Conventionally, in order to prevent the transmission characteristics due to such nonlinear distortion from being deteriorated, there have been known techniques of detecting power leaked into an adjacent channel. The detected power is used to control the transmission power so that the nonlinear distortion becomes small.
A radiophone system disclosed by Japanese Patent Publication (before examination) No. 6-77876 is known as the conventional radio system making use of the time-division multiplexing technique that controls the transmission power. This radiophone system makes use of, under transmission, power slightly leaked from a transmission/reception switchover switch that switches over between a transmitter and a receiver. Based on this detected adjacent-channel leakage power, the transmission power is controlled so that the nonlinear distortion arising in a transmission power amplifier becomes small. Thus, the intention of this system is to maintain the linearity of a transmitted output. A characteristic of selectivity of channels in this radiophone system depends on attenuation at an intermediate frequency filter incorporated in the receiver.
However, such conventional radio systems that operate on the time-division multiplexing technique have a configuration of detecting the leakage power into an adjacent channel by sensing a slight amount of leaked power from the transmission/reception switchover circuit. This causes a level of the leaked power to vary depending on various factors, such as irregularities in quality of circuit parts or changes in surrounding environments, even when the transmitted power is kept to the same amount. That is, in the case of the conventional radio systems, it is almost difficult to detect the leaked power of an adjacent channel with precision.
This lack of precision in detecting the leaked power makes it impossible to control the transmission power with precision and stability, which results in a problem that the linearity of the transmitted output can not be attained satisfactorily.
As stated above, the characteristic of selectivity of channels in the conventional time-division multiplexing radio system depends on an amount of the signal attenuated by the intermediate frequency filter. Accordingly, in a system with narrow band intervals between channels, a signal to be transmitted to the present channel can not be fully prevented from being leaked, resulting in a reduced detectable dynamic range of the adjacent channel.